Critical patch size generated by Allee effect in gypsy moth, Lymantria dispar (L.)

E Vercken,P C Tobin,A M Kramer,J M Drake

doi:10.1111/j.1461-0248.2010.01569.x

Abstract

Allee effects are important dynamical mechanisms in small-density populations in which per capita population growth rate increases with density. When positive density dependence is sufficiently severe (a ‘strong’ Allee effect), a critical density arises below which populations do not persist. For spatially distributed populations subject to dispersal, theory predicts that the occupied area also exhibits a critical threshold for population persistence, but this result has not been confirmed in nature. We tested this prediction in patterns of population persistence across the invasion front of the European gypsy moth (Lymantria dispar) in the United States in data collected between 1996 and 2008. Our analysis consistently provided evidence for effects of both population area and density on persistence, as predicted by the general theory, and confirmed here using a mechanistic model developed for the gypsy moth system. We believe this study to be the first empirical documentation of critical patch size induced by an Allee effect.

Highlights

Allee effects are a small-population phenomenon in which individual fitness increases with population density (Allee 1931)
To verify that local processes such as mating and dispersal could cause such a relationship, we developed a mechanistic model of gypsy moth population growth rate
To confirm that intrinsic processes of gypsy moth population dynamics could cause the patterns we observed, we explored the interaction between an Allee effect and random dispersal in a mechanistic model of population growth in one generation

Summary

Introduction

Allee effects are a small-population phenomenon in which individual fitness increases with population density (Allee 1931). Allee effects are always expressed as an increase in population growth rate with increased population size caused by positive demographic feedbacks (Courchamp et al 2008). The consequences of these feedbacks in locally well-mixed populations are theoretically well understood and include the creation of an interior critical point (an unstable equilibrium, the ÔAllee thresholdÕ) and the associated bistability (extinction or persistence at a density greater than the Allee threshold) characteristic of strong Allee effects (Wang & Kot 2001). Documented effects are primarily revealed as colonization patterns (Soboleva et al 2003; Davis et al 2004) or spread rates (Keitt et al 2001; Johnson et al 2006; Tobin et al 2007)

Objectives

Methods

Results

Conclusion